This invention relates to a magnetic toner for pressure fixation which is applicable to the electrophotographic process, magnetic printing process, etc.
The well known copying process for the electrophotography comprises forming an electrostatic latent image on a photosensitizer of, for example, Se, ZnO, organophotoelectric conductor, etc., developing the latent image by a magnetic developer according to a magnetic brush process, transferring the developed image onto a transfer sheet of ordinary paper, etc., and fixing the image, thereby obtaining an ultimate image. The magnetic developer so far used in the copying process is a binary developer based on a powdery mixture of a magnetic carrier and a non-magnetic toner of a fixing resin containing a coloring pigment, dyestuff, etc. However, the binary developer requires a developing apparatus provided with a means for keeping a toner concentration in the developer constant, or a means for mixing the toner with the carrier uniformly, and thus the developing apparatus becomes larger in size and complicated. These are problems with the binary developer. Furthermore, carrier particles must be mixed with toner by stirring for a long time, so that a toner film is formed on the surface of carrier particle, resulting in lowering of triboelectric charge characteristics of carrier. Thus, the carrier must be exchanged periodically.
To solve these problems, it has been proposed to use, as a magnetic developer for developing an electrostatic image, a single component magnetic toner comprising the resin and magnetic powder as the main components without using any carrier particle, and this proposal has been practically used at first in a process for direct fixation after development, using a special recording paper such as a zinc oxide paper and an electrostatic recording paper, and then also in a process for electrophotographic copying including the said transfer step. Magnetic toners are used not only in the electrophotographic process and the copying process utilizing an electrostatic recording, but also in the magnetic recording process using a magnetic drum.
In the foregoing copying processes, a heat fixation in an oven or by heat rolls, a pressure fixation only by application of a pressure at the room temperature, etc. are known to fix toner images, and the pressure fixation has been more and more used owing to no preheating time, that is, possible quick start, less power consumption, etc.
Known toners for the pressure fixation include a toner comprising an aliphatic main component of, for example, wax, etc., admixed with a thermoplastic resin for modification (Japanese Patent Publication No. 44-9880), and also a capsule type toner for pressure fixation comprising a tacky resin as nuclei, as disclosed, for example, in Japanese Patent Application Kokai (Laid-open) No. 49-17739, a magnetic toner based on wax and ethylene-vinyl acetate copolymer as resin components, as disclosed in U.S. Pat. No. 3,925,219, and a magnetic toner based on resin components of a thermoreactive resin or epoxy resin and ethylene-vinyl acetate copolymer, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 51-36947. Furthermore, a magnetic toner based on a resin obtained by modifying a waxy compound having a yield strength of 3-300 kg/cm.sup.2 with an ethylene-vinyl acetate copolymer having a definite yield strength has been practically used, as disclosed in Japanese Patent Publication No. 54-3373.
A magnetic toner for pressure fixation must have a good fixability, a good preservability at a high temperature, a good offset resistance, etc., but the so far known toners have failed to satisfy all of these requirements. For example, the toner disclosed in Japanese Patent Publication No. 44-9880 requires a high pressure at the fixation and is liable to coagulate while preserved. Microcapsule-type toners require a complicated preparatory procedure and are not practical, further because toners having stable characteristics are hardly obtainable. The toners based on a resin obtained by modifying a wax with an ethylene-vinyl acetate copolymer are more practical owing to a better fixability, stabler preservability and better flowability, but are somewhat poor still in the preservability at a high temperature and adhesion to a photosensitizer owing to a poor compatibility of the wax to other resins.